The present invention relates to the radio-frequency and microwave-assisted processing of materials, and in particular, but not exclusively, to the radio-frequency and microwave-assisted heating of ceramics, ceramic-metal composites, metal powder components, and engineering ceramics. To that end there is described a radio-frequency and microwave assisted furnace and a method of operating the same.
A hybrid furnace which combined conventional radiant and/or convective heating with microwave dielectric heating was described in the applicant""s International Patent Application No. PCT/GB94/01730 which was published under International Publication No. WO 95/05058 on Feb. 16, 1995. In addition the International application also described in detail the problems associated with the conventional firing of ceramics and glass, the problems associated with the microwave only firing of ceramics and glass and the various interactions that take place between microwaves and materials. For this reason, and in order to avoid any undue repetition, the contents of International Patent Application No. PCT/GB94/01730 are incorporated herein by reference and is to be read alongside the present specification.
Conventional radiant or convective heating heats the surface of a sample and relies on thermal conduction to transfer heat from the surface throughout the volume of the sample. If a sample is heated too quickly, temperature gradients are produced which can lead to thermal stress and, ultimately, to the failure of the material. As the size of the sample is increased, this effect becomes exaggerated and, generally, samples have to be heated more slowly as their dimensions are increased.
The presence of temperature gradients also means that the whole of the sample cannot be processed using the same temperature-time schedule. This in turn often leads to variations in microstructure (eg grain size) throughout the sample, and, since not all parts of the sample can be processed to the optimum extent, poorer overall properties such as density, strength etc.
By contrast, careful balancing of conventional surface heating and microwave heating (ie volumetric heating) can ensure that the whole sample is heated uniformly without giving rise to temperature gradients and so leading to the possibility of much more rapid heating (particularly where large samples are concerned) without the risk of thermal stresses developing. Furthermore, since the whole sample can be processed to an optimum temperaturexe2x80x94time schedule, it is possible to produce a highly homogeneous microstructure of increased density and increased material strength. It was this method of controlling the relative quantities of surface and volumetric heating that formed the subject of the applicant""s earlier International Patent Application No. PCT/GB94/01730.
In addition to the thermal benefits produced by the volumetric nature of microwave heating, there is also increasing evidence to support the presence of a so-called non-thermal microwave effect during sintering. This is an effect which would not be observed even if conventional heat could somehow be introduced to the sample in the same volumetric way as microwave energy. Samples processed within a microwave furnace are observed to sinter at a faster rate or at a lower temperature than those processed in a conventional system. For example, Wilson and Kunz described in J.Am. Ceram. Soc 71(1) (1988) 40-41 how partially stabilised zirconia (with 3 mol % yttria) could be rapidly sintered using 2.45 GHz microwaves with no significant difference in the final grain size. The sintering time was reduced from 2 hours to about 10 minutes. This has been explained with reference to an effective activation energy for the diffusion processes taking place during sintering so that, for example, Janney and Kimrey describe in Mat. Res. Symp. Proc. Vol. 189 (1991), Materials Research Society that at 28 GHz, the microwave enhanced densification of high purity alumina proceeds as if the activation energy is reduced from 575 kJ/mol to 160 kJ/mol.
Despite the potential implications for the ceramics industry the physical mechanisms which give rise to this effect are not understood. The microwaves must interact with the ceramic so as to either reduce the actual activation energy or increase the effective driving force experienced by the diffusing species. Both possible mechanisms have their supporters but the present applicant favours the existence of an enhancement to the driving force. This at least is consistent with the calculations of Rybakov and Semenov who showed in Phys. Rev. B.49(1) (1994) 64-68 that the driving forces for vacancy motion can be enhanced near a surface or boundary in the presence of a high frequency electric field.
The power density, Pv, dissipated within a sample heated by a microwave field is given by
Pv=2xcfx80fxcex5oxcex5rxe2x80x3E2 xe2x80x83xe2x80x83(1) 
where f is the frequency of the applied field, xcex5o is the permittivity of free space, xcex5rxe2x80x3 is the dielectric loss factor of the material, and E is the electric field strength. Rearranging this equation the electric field is given by                     E        =                                            P              v                                      2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              xe2x80x83                            ⁢                              ϵ                o                            ⁢                              ϵ                r                xe2x80x3                                                                        (        2        )            
Unfortunately, the dielectric loss factors of many low loss ceramic materials such as alumina, zirconia etc increase almost exponentially with increasing temperature. Assuming that the power density required for heating remains constant during the process, equation (2) implies that the electric field strength in the material must fall away rapidly with increasing temperature. Consequently, the magnitude of any non-thermal effects due to the presence of the electrical field will also be reduced at higher temperatures just when the diffusing species are most free to move through the material since the diffusion coefficient increases exponentially with increasing temperature.
Similarly, the depth of penetration (ie the distance in which the power density falls to 1/e of its value at the surface) for electromagnetic waves such as microwaves propagating in a dielectric material is given by                               d          p                =                                            c                              2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                f                ⁢                                  2                                ⁢                                  ϵ                  r                  xe2x80x2                                                      ⁡                          [                                                                    1                    +                                                                  (                                                                              ϵ                            r                            xe2x80x3                                                                                ϵ                            r                            xe2x80x2                                                                          )                                            2                                                                      -                1                            ]                                            1            2                                              (        3        )            
where xcex5rxe2x80x2 is the dielectric constant of the material and c is the speed of light in a vacuum. If one were to consider yttria stabilised zirconia (8%YSZ), at low temperatures (ie at approximately 200xc2x0 C.) and at 2.45 GHz, a standard microwave frequency, the dielectric constant, xcex5rxe2x80x2, is approximately 20 and the dielectric loss factor, xcex5rxe2x80x3, is about 0.2. Inserting these values into equation (3) gives a penetration depth of 45 cm. At higher temperatures of approximately 1,000xc2x0 C., xcex5rxe2x80x2 is approximately 34 and xcex5rxe2x80x3 is approximately 40, giving a penetration depth of only 0.3 cm. Thus at high temperatures microwaves of 2.45 GHz are not particularly effective at heating samples of yttria stabilised zirconia of more than about 1 cm thick, although this is still much better than conventional methods of heating where only the immediate surface is heated. Again, however, any non-thermal microwave effect will also be limited to the penetration depth.
In order to overcome these problems whilst making the optimum use of any non-thermal effect, according to a first aspect of the present invention there is provided a hybrid furnace comprising a microwave source, an enclosure for the confinement of both microwave and RF energy and for containing an object to be heated, means for coupling the microwave source to said enclosure, an RF source, means for coupling the RF source to said enclosure, and control means for controlling the quantity of microwave energy and RF energy to which the object to be heated is exposed.
Advantageously, the hybrid furnace may additionally comprise radiant and/or convective heating means disposed in relation to the enclosure to provide radiant and/or convective heat as appropriate within the enclosure and means for controlling the quantity of heat generated in the object by the radiant and/or convective heat.
According to a second aspect of the present invention there is provided a method of operating a furnace of the type comprising a microwave source, an enclosure for the confinement of both microwave and RF energy and for containing an object to be heated, means for coupling the microwave source to said enclosure, an RF source, and means for coupling the RF source to said enclosure, the method comprising the steps of actuating the microwave source to heat the object and actuating the RF source to provide an oscillating electric field within the object to be heated at a location and/or at a temperature where the field strength of the microwave-induced electric field falls below a predetermined threshold value.
Advantageously, the furnace may additionally comprise radiant and/or convective heating means and the method may then comprise the additional steps of actuating the radiant and/or convective heating means so as to generate radiant and/or convective heat substantially throughout the heating cycle of the object and controlling the quantity of heat generated in the object by one or both of the microwave energy and the radiant and/or convective heat so as to provide a desired thermal profile in the object.
Radio-frequency (RF) is another form of dielectric heating involving a high frequency electric field and is also described by equations (1) to (3). However, radio-frequencies are much lower than those of microwavesxe2x80x94typically 13.56 MHz (ie a factor of 181 times less than 2.45 GHz). Thus, for the same values of xcex5rxe2x80x3 and Pv, equation (2) suggests that the electric field will be 13 times higher for the RF case than for the microwave case. Indeed, the dielectric loss factors of ceramics at radio-frequencies are usually much smaller than at microwave frequencies so that in fact the electric field will be even higher.
Likewise, an inspection of equation (3) reveals that the penetration depth is proportional to 1/f. Consequently, assuming that all other parameters are the same, dp will be 181 times larger in the RF case than in the microwave case and the resulting electric field will penetrate deep within the material even at very high temperatures.
Unfortunately, many ceramic materials are not heated effectively when they are placed solely in an RF electric field. The required electric field to give reasonable energy dissipation at this frequency is often in excess of that which would cause electrical breakdown in the furnace. However, by providing a hybrid system which uses both microwave and RF volumetric heating this problem can be overcome. When combined with conventional surface heating techniques even greater benefits may be obtained.